I've found that serials operate according to a slightly different set of rules than regular novels. I wrote fantasy gaming stories at EnWorld (here's an example) for about eight years. The trick (at least for me) was to have a lot of cliffhangers and characters that readers could develop relationships with. In fantasy there are a lot of immediately-recognizable archetypes that make this easier to do. You don't necessarily have to have your characters in a life-or-death situation at every break point but it helps to have a reason for your readers to come back for the next installment. And your first post has to have something in it that snags the reader; I suppose it's not that different from the first few pages of a novel in that respect, but given the post length you cited in essence you are selling the entire story in a very compact introduction. I tried to make my posts about 1.5-2k words and that worked well for me, but then again my stories were much longer (the one I linked above ended up being 633k words).

There is no need to send anyone off to look elsewhere! MR has its very own serial novel(la) happening right now (you must have noticed The murder mystery thread). What better example could we look for? Murder and mayhem in every installment.